White toner particles and their use is known in the art, especially for printing on a black background or for printing techniques wherein black and white toners are used for printing different gray levels.
In EP-A-253 560, an electrophotographic copying method is disclosed, comprising the use of a copying apparatus having a positive-image copying system, wherein toner images with a colored toner are formed and transferred to a colored insulating paper of a color different to that of said toner while selecting the colors of the paper and the toner so as to form reverse images. In an example a white toner with 100 parts by weight of a styrene-acrylic resin and 20 parts by weight of rutile type TiO.sub.2 is disclosed.
In EP-A-280 378 a white toner is disclosed comprising a fixing resin and dispersed therein, a titanium dioxide pigment of high purity containing at least 99% by weight of TiO.sub.2, not more than 0.1% by weight of Al.sub.2 O.sub.3 and not more than 0.05% by weight of SiO.sub.2 as a white pigment, said titanium dioxide having an average particle diameter of not less than 0.05 .mu.m. In this disclosure it is stressed that good charging quality of the toner particles can not be reached when the TiO.sub.2 is not that pure. It is said that maximum 50 parts by weight, preferably maximum 30 parts by weight, of the TiO.sub.2 can be used for 100 parts by weight of toner resin because toner with higher amounts of TiO.sub.2 can not be fixed.
In JP-A-01 048067 a white toner is disclosed wherein between 5 and 20% by weight of TiO.sub.2 is present, preferably there is between 8 and 10% by weight of TiO.sub.2.
In U.S. Pat. No. 4,943,506 a white toner is disclosed comprising binder resin and titanium dioxide with 0.20-0.35 .mu.m in mean particle size at the content of 15-60 parts by weight on the basis of 100 parts by weight of the binder resin.
In U.S. Pat. No. 5,077,158, a process for forming an image is disclosed, which comprises developing an electrostatic latent image with a gray toner, wherein a mixture of a white toner and a black toner is used as the gray toner. The white toner comprises preferably TiO.sub.2 in the rutile crystal structure and for at most 50 parts by weight for 100 parts by weight of toner resin.
Nowadays a well accepted way of printing labels is printing them with digital electrostatographic printing means, e.g. the CHROMAPRESS (trade name of Agfa-Gevaert NV, Mortsel, Belgium) or the DCP1 (trade name of Xeikon NV, Mortsel, Belgium). Also transparent labels are printed with such printing devices, and when opaque white images on a transparent support, it is preferred to have toner particles giving a high opacity, i.e. the hiding power of a layer of white toner must be very high. When a transparent self-adhesive label for application on a colored, e.g. red, background is printed with white lettering, it is desired that the lettering really shows white and not white with a faint red hue. Also when on a transparent label a bar-code has to be printed, it is desired for better readability of the bar-code that it is printed on a white background and then the white must be very opaque. The known toner particles are well suited for printing white images, but for printing for the printing of white images on transparent substrates, the hiding power has to be still higher.